Capacity limits for single-mode fiber transmission has been a subject of research ever since it was recognized that the Kerr nonlinearity imposes a fundamental limit on fiber capacity. It is well known that the nonlinear coefficient is inversely proportional to the effective area in a single-mode fiber. Therefore, a simple and effective way to reduce the nonlinear penalty is to increase the fiber core diameter and thus enlarge the effective area. However, this approach is limited by increased macro-bending loss and/or dispersion.
Recently, a new method using “few-mode fibers” in single-mode operation was proposed to increase the core diameter without changing the loss and dispersion properties. Few-mode fibers (FMF) are optical fibers that support more than one spatial mode but fewer spatial modes than conventional multi-mode fibers. Although FMFs can carry more than one mode, the fundamental mode can be excited and transmitted without mode coupling over very long distances, as long as the effective indexes of the supported modes are sufficiently different from each other.
Space-division multiplexing (SDM) has also been proposed to increase fiber capacity. Fiber bundles are attractive for use in SDM because of their simplicity and compatibility. FMF is also a candidate for use in SDM because it supports a few large effective area modes and because mode coupling can be avoided if a large effective index difference (ΔNeff) exists among the modes. Unfortunately, there are some drawbacks associated with using FMFs for long-distance SDM. First, there is typically a large differential modal group delay (DMGD) among the modes of FMFs. Second, the modal loss of FMFs increases with mode order. Third, mode coupling is inevitable when using FMFs as the number of modes increases because large effective index differences are difficult to maintain for all modes.
Multi-core fiber (MCF) has further been proposed as a candidate for SDM due to its zero DMGD, equal loss, and ultra-low crosstalk between modes. However, the mode density of MCFs typically must be kept quite low in order to maintain low crosstalk. For example, the first MCF demonstrated for SDM transmission was painstakingly fabricated to reduce the crosstalk to a level of less than −30 dB/km. In more recent efforts, crosstalk has been reduced to the current record of −90 dB/km. In addition, each mode of an MCF still suffers a large nonlinear penalty because its effective area is the same as or smaller than that of an SMF.
As can be appreciated from the above discussion, it would be desirable to have a high-capacity optical transmission approach that avoids one or more of the above-described drawbacks.